1. Field of the Invention
The present invention relates generally to image processing and more specifically pertains to a video compression algorithm for increasing the data throughput on a limited bandwidth link between a host and a digital video camera.
2. Description of Related Art
A digital image represents a two-dimensional array of samples, where each sample is called a pixel. Precision determines how many levels of intensity can be represented and is expressed as the number of bits/sample. Resolution of an image refers to its capability to reproduce fine details. Higher resolution requires more complex imaging systems to represent these images in real-time. In video systems, resolution refers to the number of line pairs resolved on the face of the display screen, expressed in cycles per picture height, or cycles per picture width. Full motion video is characterized with at least 24-Hz frames/sec, and 30 or even 60 frames/sec for high definition TV. For animation, acceptable frame rate is in the range of 15-19 frames/sec while for video telephony it is 5-10 frames/sec. Videoconferencing and interactive multimedia applications require the rate of 15-30 frames/sec.
Commercial imaging applications are surfacing in digital broadcast television, compact disk video, multimedia, video teleconferencing systems, dynamic medical imaging devices, and high definition TV. Imaging applications are time critical and computationally and data intensive, and require both the storage and transmission of enormous data sets which can be accomplished with image compression. Achieving the compression ratios necessary for digital video involves the processing of individual images to remove spatial redundancies and a motion analysis of the sequence to remove temporal redundancies.
The latest models of still and video cameras are solid-state cameras which use silicon chips for image data arrays. Since the size of the camera and the chip should be kept small, these cameras have to keep low the number of sampled image data transferred from the camera to an accompanying host image processing device. Furthermore, the mass consumers of the digital video cameras are used to viewing video images on TV at the frame rate of 30 frames per second. The currently available buses between digital video cameras and the host system have a limited bandwidth of less than 8 frames per second, which limits the number of video frames that can be transferred from the camera to the host and provides fuzzy pictures. These buses do not allow real-time video data transmission.
In order to attract the average user to purchase a digital camera, the throughput of data through a low bandwidth transmission channel between the camera and the host has to be improved by compression. The real time image processing presently involves at least three types of tradeoffs: performance versus image resolution, performance versus storage size, and performance versus input and output bandwidth. The quality of performance is influenced by the cost. Expensive compression schemes, such as MPEG, are not cost effective for low cost single chip cameras. The basic measure for the performance of a compression algorithm is compression ratio, defined as a ratio of original data size and compressed data size. There is a trade-off between the compression ratio and the picture quality. Higher compression ratios may produce lower picture quality. Quality of compression can also vary according to source image characteristics and scene content.
Therefore, there is a need for a low cost method which can improve the data bandwidth between the digital video camera and the host by applying video compression to the digital video stream before the video frame is transmitted to the host computer, which is capable of generating sharp, high quality color images in real time. Furthermore, there is a need for small and inexpensive digital solid-state video cameras capable of transferring 30 video frames per second. These cameras have to have a low gate count and low power requirements so that the size and cost of the camera is minimal.